Interparticle interaction effect on the internal magnetic structure of magnetite core-shell nanoparticles

ENIO LIMA JR.; J M VARGAS; R D ZYSLER; R COHEN; H R RECHENBERG

Roma

Conferencia; ICFPM07 - 6th International Conference on Fine Particle Magnetism (ICFPM07), New trends in nanoparticle magnetism; 2007

Magnetic nanoparticles with narrow size distribution and coated by surfactant molecules are preapred by high temperature chemical synthesis. According to TEM micrographs, as-made samples is composed by nanoparticles with mean diameter of 9 nm. The internal particles structure is constituted by a crystalline core and disordered shell (~50 % vol.). The as-made particles were studied in two distinct aggregation configuration with different interparticle interaction strenght: non-interacting particles dispersed in a polymer, and relatively strong interacting particles in a powder samples. M(H,T) measurements clearly show that the interparticle interactions modify spin structure of the shell, inducing a short range magnetic order in the surface of interacting nanoparticles, which is thermally stable. In-field Mössbauer (up to 120 kOe) analysis clearly indicate that surface effects play the fundamental rule in the magnetic properties of the system. On the other hand, annealed sample is composed by well-crystalline 6 nm nanoparticles. In other to compare the magnetic properties of these particles with the as-made nanoparticles (core-shell structured) the annealed particles were embedded in a polymer. M(H,T) and Mössbauer measurements show that the magnetic properties are dominated by magno-crystalline anisotropy as resulted from the very high crystallinity degree of the nanoparticles. Moreover, the saturation magnetization of the annealed sample increases five times with respect to the as-made sample, arriving a value only 38 % smaller than the bulk magnetite saturation magnetization.